The remnants of star death in the western sky

Aileen O’Donoghue

We humans look at the stars and constellations as eternal. But even stars are born, live for some time and die. Of course these stages occur over time scales much, much longer than human lifetimes. And yet, we puny humans, crouched in the dark on this tiny dust-ball of a planet peering through telescopes and spectroscopes, have manage to discern the complex processes of stellar lives.

Our most familiar star-birth location is the Great Nebula in Orion, the fuzzy blur forming part of the sword. I’ll leave that and other star formation objects for later columns. Most stars, like our Sun, are in midlife, which astronomers call the “main sequence.” These stars shine brightly as they fuse hydrogen into helium in their cores. The fractional size of the core within the star where the temperatures (~15 billion C) and densities (160 g/cm3) are great enough for fusion is about 0.25 (25 percent) in the Sun and proportionally larger or smaller in stars of greater or lesser mass, respectively. Hence large stars use up their fuel quickly, within tens of millions of years. The sun will fuse hydrogen for about 10 billion years and less massive stars have longer lives. The universe is too young for any star smaller than 0.8 solar masses to have lived out its main sequence life.

As their hydrogen cores are depleted, stars must either fuse heavier elements or die. The Sun will eventually manage to fuse some helium into carbon, but will not be hot or dense enough to fuse heavier elements. Very large stars, more than five times the mass of the Sun, fairly easily transition into fusing heavier and heavier elements until they create a core of iron. For all stars, the end of the initial hydrogen fusion instigates their death throes and they will soon (in astronomical terms … a million to a billion years) die and enrich the surrounding space with the products of their fusion.

Helium builds up in stellar cores during main sequence life. As the helium replaces the hydrogen fuel in the core, the nuclear fires abate, allowing the core to cool and consequently contract. The core, already very dense and under immense pressure due to the weight of the outer layers of the star, becomes a very weird state of matter called a degenerate electron gas with densities up to 10,000 kg/cm3 (that’s 10 metric tons per cubic centimeter!). But, the shrinking brings more hydrogen into the region hot and dense enough for fusion, and re-ignites a spherical shell of hydrogen surrounding the shrinking helium core. This heats the outer envelope more intensely than the normal fusion core did, because the degenerate core can’t expand to spend any energy as normal gases do. Hence the outer layers absorb more of the heat and expand to immense sizes. The Sun at this stage will become nearly the size of Earth’s orbit, whereas it’s currently about 1 percent of Earth’s orbit. Also, since the outer layers expand, they cool to create a red giant star. Such stars are visible all over the sky, including Erakis (Ear-RACK-iss) in Cepheus that’s an astounding 1,650 times larger than the Sun. If it replaced the Sun, its outer layers would be between the orbits of Jupiter and Saturn. This star, also known as Hershel’s Garnet star for the deep red color visible in binoculars, is probably fusing helium into carbon. As you breathe, it is long chains of carbon that give your lungs the flexibility to expand and contract. Most of those carbon atoms were forged in red giant stars squeezing the last out of their fuel at the ends of their lives.

Eventually, when the stellar core can fuse no more, the distended outer layers of the star continue to expand away from the emerging (degenerate) core still hot from the fires of fusion. The hot core sheds particles in fierce stellar winds that blast through the outer layers, blowing them away from itself. The core will remain as a white dwarf as the outer layers become a planetary nebula. The name is due to the angular extent making them look like planets in the first telescopic views. We now know these stellar corpses come in a variety of shapes, but dominated by spheres and ellipsoids. As shown in Figure 1 by the circles with horizontal and vertical fins, they are abundant in the night sky showing that many Sun-sized stars have already met their fates. The carbon in your lungs, and every one of your organs has been in one of these beautiful objects.

An image of the Cat’s Eye Nebula in Draco shown in Figure 2 has a “bullseye” appearance due to successive outbursts of material about every 1,500 years. The inner part also shows multiple bubbles of expelled material. The carbon forming your eyes was once part of one of these magnificent objects. You read this through lenses of stardust. A quick Google search on any of the names of in Figure 1 will reveal other beautiful images in stunning colors.

Large stars that have built up iron cores die much more quickly and spectacularly. The iron core absorbs energy and begins cooling and shrinking as soon as it forms. It, too, becomes a degenerate electron gas, but the outer layers of the star continue to fuse more iron, increasing the mass of the core. When it reaches a mass 1.4 times that of our Sun, it can’t hold itself up against gravity and a thermonuclear explosion of the core and the entire star becomes a supernova. These are Type II supernovae and the oxygen you’re breathing was likely released in one of these.

Other supernovae occur when one star in a binary pair evolves into a white dwarf orbiting a larger companion. Material from the companion star can be pulled onto the white dwarf until it exceeds 1.4 solar masses and explodes as a Type 1a supernova.

These have been observed from Earth throughout time. Four historical supernova remnants are shown in Figure 1. Tycho’s supernova was the most important of these. When a “new star”, a Type Ia supernova, suddenly appeared in 1572, astronomers took notice. Its appearance challenged the accepted notion of “unchanging eternal heavens” much to people’s discomfort. For Galileo, this, the mountains he saw on the moon, the phases he observed Venus going through as it orbited the Sun, contributed to his acceptance of new models of the heavens based on observations instead of scriptures.

The Victory Star was also a Type Ia supernova and observed in June 1918, outshining all stars but Sirius in the northern sky. Some took it as a good omen as WWI struggled toward a close. SN 1181 was observed and recorded by Chinese and Japanese astronomers in 1181. It was likely a Type II supernova.

Cassiopeia A was actually discovered in a radio study in 1948. Further observations have revealed it to have been a Type II supernova. Since Cassiopeia never sets for northern observers, it should have been visible for astronomers around 1680. But it was probably hidden by dust surrounding the progenitor star and clouds between it and Earth. It remains a bright radio source of considerable interest.

The volunteer astronomers at the Adirondack Public Observatory are eager to show you remnants of stellar deaths as well as regions of stellar birth and other celestial delights. The Roll Off Roof Observatory (RORO) is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.